In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are sealed toward the surface of the earth by impermeable top layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks may, for example, have a diameter of only approx. 1 μm. As well as mineral oil, including fractions of natural gas, a deposit also comprises water with a greater or lesser salt content.
In mineral oil production, a distinction is drawn between primary, secondary and tertiary production.
In primary production, after commencement of drilling of the deposit, the mineral oil flows of its own accord through the borehole to the surface owing to the autogenous pressure of the deposit. The autogenous pressure can be caused, for example, by gases present in the deposit, such as methane, ethane or propane. The autogenous pressure of the deposit, however, generally declines relatively rapidly on extraction of mineral oil, such that usually only approx. 5 to 10% of the amount of mineral oil present in the deposit, according to the deposit type, can be produced by means of primary production. Thereafter, the autogenous pressure is no longer sufficient to produce mineral oil.
After primary production, secondary production is therefore typically used. In secondary production, in addition to the boreholes which serve for the production of the mineral oil, known as the production boreholes, further boreholes are drilled into the mineral oil-bearing formation. These are known as injection boreholes, through which water is injected into the deposit (known as “water flooding”), in order to maintain the pressure or to increase it again. As a result of the injection of the water, the mineral oil is gradually forced through the cavities in the formation, proceeding from the injection borehole, in the direction of the production borehole. However, this works only for as long as the cavities are completely filled with oil and the more viscous oil is pushed onward by the water. As soon as the mobile water breaks through cavities, it flows on the path of least resistance from this time onward, i.e. through the channel formed, and no longer pushes the oil onward. By means of primary and secondary production, therefore, generally only approx. 30 to 35% of the amount of mineral oil present in the deposit can be produced.
After the measures of secondary mineral oil production, measures of tertiary mineral oil production (also known as “Enhanced Oil Recovery (EOR)”) are therefore also used to further enhance the oil yield. This includes processes in which particular chemicals, such as surfactants and/or polymers, are used as assistants for oil production. An overview of tertiary oil production using chemicals can be found, for example, in the article by D. G. Kessel, Journal of Petroleum Science and Engineering, 2 (1989) 81-101.
The techniques of tertiary mineral oil production include what is known as “polymer flooding”. Polymer flooding involves injecting an aqueous solution of a thickening polymer through the injection boreholes into the mineral oil deposit, the viscosity of the aqueous polymer solution being matched to the viscosity of the mineral oil. As a result of the injection of the polymer solution, the mineral oil, as in the case of water flooding, is forced through the cavities mentioned in the formation, proceeding from the injection borehole, in the direction of the production borehole, and the mineral oil is produced through the production borehole. By virtue of the fact that the polymer formulation, however, has about the same viscosity as the mineral oil, the risk is reduced that the polymer formulation breaks through to the production borehole with no effect, and hence the mineral oil is mobilized much more homogeneously than in the case of use of mobile water. It is thus possible to mobilize additional mineral oil in the formation.
For polymer flooding, a multitude of different thickening polymers have been proposed, especially high molecular weight polyacrylamide, copolymers of acrylamide and further comonomers, for example vinylsulfonic acid or acrylic acid. Polyacrylamide may especially be partly hydrolyzed polyacrylamide, in which some of the acrylamide units have been hydrolyzed to acrylic acid. In addition, it is also possible to use naturally occurring polymers, for example xanthan or polyglycosylglucan, as described, for example, by U.S. Pat. No. 6,392,596 B1 or CA 832 277.
Another of the techniques of tertiary mineral oil production is known as “surfactant flooding”. Surfactant flooding involves injecting suitable surfactants into the mineral oil deposit through the injection borehole. The surfactants lower the interfacial tension σ between the mineral oil and the aqueous phase. Mineral oil remaining in the mineral oil formulation forms a micro-emulsion with the surfactants, such that it can now also be forced out of the formation through fine capillary openings by injected flooding water.
Details of techniques for tertiary mineral oil production such as “surfactant flooding” and “polymer flooding” for example are disclosed, for example, in “Petroleum, Enhanced Oil Recovery, Kirk-Othmer, Encyclopedia of Chemical Technology, online edition, John Wiley & Sons, 2010”.
As already outlined above, the case may occur in water flooding that the mineral oil deposit is exploited inhomogeneously. In more pervious regions of the mineral oil formation, the reservoir is exploited to a greater degree, and there may be a breakthrough of the water to the production borehole.
When the technique of surfactant flooding is applied to such mineral oil deposits, there is the risk that the surfactant solution also flows through more pervious regions which therefore have lower flow resistances from the injection borehole to the production borehole, and mineral oil is not mobilized by the surfactant to the desired degree. It is therefore known that surfactant solutions can be thickened with thickening polymers in order to prevent the surfactant solution from flowing preferentially through the more pervious regions of the mineral oil deposit.
In addition to the abovementioned thickening polymers, hydrophobically associating copolymers can also be used for polymer flooding. “Hydrophobically associating copolymers” are understood by the person skilled in the art to mean water-soluble polymers which have lateral or terminal hydrophobic groups, for example relatively long alkyl chains. In aqueous solution, such hydrophobic groups can associate with themselves or with other substances having hydrophobic groups. This forms an associative network by which the medium is thickened. Details of the use of hydrophobically associating copolymers for tertiary mineral oil production are described, for example, in the review article by Taylor, K. C. and Nasr-El-Din, H. A. in J. Petr. Sci. Eng. 1998, 19, 265-280.
EP 705 854 A1, DE 100 37 629 A1 and DE 10 2004 032 304 A1 disclose water-soluble, hydrophobically associating copolymers and the use thereof, for example in the construction chemistry sector. The copolymers described comprise acidic monomers, for example acrylic acid, vinylsulfonic acid, acrylamidomethylpropanesulfonic acid, basic monomers such as acrylamide, dimethylacrylamide, or monomers comprising cationic groups, for example monomers having ammonium groups, and also monomers which can bring about the hydrophobic association of the individual polymer chains.
In the case of combination of hydrophobically associating polymers with surfactants, the viscosity of the solution obtained from polymer and surfactant, according to the type of interactions between the surfactants and the polymers, may be either greater or smaller than the viscosity of the polymer solution alone (see, for example, W. Zhou, Y Guo, M. Dong, H. Xiao, Journal of Canadian Petroleum Technology, 43(2), 2004, pages 13 to 16). For use for tertiary mineral oil production, a decrease in the viscosity is of course highly undesirable because more polymer has to be used to attain the desired viscosity, and this greatly impairs the economic viability of the process.
U.S. Pat. No. 4,814,096 discloses a mixture of hydrophobically associating polymers and surfactants for tertiary mineral oil extraction. The hydrophobically associating polymer comprises hydrophilic monomers, for example acrylamide. The hydrophobic monomers are C8- to C20-alkyl esters of ethylenically unsaturated carboxylic acids, N-alkyl derivatives of ethylenically unsaturated carboxamides with C8- to C20-alky radicals, vinyl alkylates of alkyl radicals having at least 8 carbon atoms and alkylstyrenes having alkyl radicals of at least 4 carbon atoms. The surfactants are nonionic surfactants and the ratio of polymer to surfactant is preferably 20:1 to 0.5:1.
GB 2 199 354 A1 discloses a mixture of hydrophobically associating polymers and surfactants for secondary mineral oil production. The polymer comprises acrylamide, monomers having sulfo groups and N-alkyl- or N,N′-dialkylacrylamides, where at least one of the alkyl radicals is a C6- to C22-alkyl radical. The polymer is used in an amount of 0.0001 to 0.5% by weight and the surfactant in an amount of 0.1% by weight to 5% by weight.
US 2010/0197529 A1 discloses a further mixture of hydrophobically associating polymers and surfactants for secondary mineral oil production. The polymer is notable in that it comprises a hydrophobic cationic monomer, preferably ethylenically unsaturated trialkylammonium derivatives, where at least one of the alkyl radicals is a C8- to C30-alky radical. The weight ratio of polymer to surfactant is 1:1 to 1:10.
Our prior application WO2010/133527 A2 discloses hydrophobically associating copolymers which comprise at least hydrophilic, monoethylenically unsaturated monomers, for example acrylamide, and monoethylenically unsaturated, hydrophobically associating monomers. The hydrophobically associating monomers have a block structure and have—in this sequence—an ethylenically unsaturated group, optionally a linking group, a first polyoxyalkylene block which comprises at least 50 mol % of ethyleneoxy groups, and a second polyoxyalkylene group which consists of alkyleneoxy groups having at least 4 carbon atoms. The application discloses the use of such copolymers as thickeners, for example for polymer flooding, for construction chemical applications or for detergent formulations.
Our prior application WO2011/015520 A1 discloses a process for preparing hydrophobically associating copolymers by polymerizing water-soluble, monoethylenically unsaturated surface-active monomers and monoethylenically unsaturated hydrophilic monomers in the presence of surfactants, and the use of such copolymers for polymer flooding.
Both applications further disclose that the hydrophobically associating copolymers disclosed can be mixed with surfactants, without disclosing details with respect to the surfactants and the mixtures.